Frequency is a derived SI unit but its unit is 1/s. It uses only time once and no other fundamental quantities. So why is not included in SI base unit?
They try to keep the base units to as few as possible. If it can be expressed wholly in terms of other base units, it is derived. Of course, they could have chosen Hz to be the base unit, and then s would have been derived.
Base unit is s. Unit of frequency is 1/s so it is a derived unit which is commonly called Hz.